The providers of certain multiple-access communications systems, such as wireless cellular mobile telephone (CMT) and personal communication systems (PCS), prefer to employ base station transmitter and receiver equipment that is as flexible as possible in terms of the channel coverage provided by a particular transceiver site. This is true for CMT systems deployed in rural areas, where signal traffic may be concentrated along a roadway, as well as for systems deployed in densely populated areas, where a fixed-in-advanced channel capacity may be inadequate.
Such characteristics are also desirable whenever relatively large, secure, and protective structures are not necessarily available or cost effective. For example, in certain PCS systems as now being proposed, a large number of small coverage areas, or cells, will be necessary. In these PCS system, wireless base station equipment may be deployed in cells as small as 500 feet in diameter.
One way to resolve these difficulties is to implement a base station transceiver using high speed analog-to-digital (A/D) and digital-to-analog (D/A) converters, together with efficient digital filtering algorithms such as the Fast Fourier Transform (FFT). In the receiver section, a forward FFT-based filter bank analyzer, or so-called channelizer, separates the incoming signal energy into multiple ones of the desired channels. On the transmit side, an inverse FFT-based filter bank synthesizer, or so-called combiner, outputs a composite frequency-modulated signal representative of the contents of the combined channel signals. In this manner, relatively compact, lightweight, inexpensive, and reliable digital integrated circuits may be used to cover the entire channel capacity offered by the communication service provider.
For a more detailed description of such a system, please refer to a copending U.S. patent application of Carney, R., and Williams, T., entitled "Transceiver Apparatus Employing Wideband FFT Channelizer with Output Sample Timing Adjustment and Inverse FFT Combiner for a Multichannel Communication Network" filed Apr. 8, 1994 and given Ser. No. 08/224,754.
Unlike prior art base stations, such a wideband digital base station is capable of receiving or transmitting on any number of channels at any instant in time. While this provides maximum flexibility and a certain number of other advantages in designing a multichannel wireless communication system, it also poses a number of unique problems.
In particular, consider that the individual signals may use modulation schemes that do not exhibit a constant amplitude envelope, such as amplitude modulation (AM), or quadrature amplitude modulation (QAM). Even when constant envelope modulation schemes, such as frequency modulation (FM), frequency shift keying (FSK), or phase shift keying (PSK) are used on the individual channel signals, the composite waveform generated by the combiner cannot be guaranteed to exhibit a constant envelope. This is because the composite signal generated is a sum of digital channel signals having non-deterministic phases, and because the channel signals may be activated independently of each other. The composite signal thus does not exhibit a constant amplitude envelope over time, regardless of the modulation used. Rather, at best, the composite signal can be predicted to have a uniformly random phase distribution and a Rayleigh probability density amplitude envelope.
The resulting Rayleigh-distributed envelope has undesirable high peak-to-average power requirements that place demanding linearity and dynamic range requirements on the transmit signal path. These requirements are especially acute for the high-power amplifier component which must typically be placed between the up converter which follows the combiner and the antenna. Because of this phenomenon, for example, a 50 Watt transmit amplifier may actually be required to handle 250 Watt signals for short periods of time. That is, the instantaneous peak power output must typically be about five (5) times the average power output, and the power amplifier design must achieve this with minimal distortion to the individual channel signals and without creation of in-band or out-of-band spurious tones.
While it is possible to design a power amplifier which has sufficient linearity to achieve this result, the amplifier is decidedly more complex in design, less efficient, and more expensive than would otherwise be required.
It would be preferable if the signal to be transmitted had an amplitude envelope with as constant a probability, density as possible. This would then permit a lower cost, non-linear power amplifier to be used. Indeed, if it were possible to allow for absolutely no variations in transmit power, that is, if an exactly constant envelope signal could be provided, then the power amplifier could be designed to run in a saturation region, and simply hard-limit the transmit waveform.
In addition, there should not be any undesirable residue frequencies, or spurious tones, created which would otherwise not be removed prior to signal transmission. Although unused in-band frequencies might be permitted to contain residual signal artifacts by design, it is unlikely that regulatory agencies would approve of the use of such techniques.